This invention relates to improved beryllium aluminum alloys for use in investment casting.
As described in commonly-assigned U.S. Pat. No. 5,667,600 to Grensing et al., the disclosure of which is incorporated herein by reference, investment casting is a type of casing normally used to make metal parts of complex shape. “Investment casting” connotes that the casting obtained has a “near net shape,” i.e., a shape which is very near to the shape of the final product to be made. Investment casting is desirable because it essentially eliminates the extensive machining that would otherwise be necessary to transform a casting into its final desired shape.
Beryllium and aluminum have widely different melting temperatures, 1289° C. and 660° C. respectively. This makes investment casting of beryllium aluminum alloys very difficult, because this large difference in melting temperatures leads to large differences between the liquidus and solidus temperatures of these alloys. See, U.S. Pat. No. 5,603,780 to Nachtrab et al., the disclosure of which is also incorporated herein, especially col. 1, lines 31 to 50. This, in turn, often leads to excessive porosities, coarse microstructures, or both in castings made from these alloys. Accordingly, shaped parts made from beryllium aluminum alloys are most commonly made by powder metallurgy techniques.
The above-mentioned Grensing et al. patent indicates that beryllium aluminum alloys suitable for investment casting can contain as little as 30 wt. % beryllium. However, experience has shown that beryllium contents of at least about 56 wt. % and more commonly about 61 to 69 wt. % are necessary to make commercially-acceptable alloys, i.e., alloys exhibiting acceptable levels of segregation and microporosity. For example, the three beryllium aluminum investment casting alloys available on the market today have beryllium contents of 56 to 68 wt. %. Note, also, that the alloys in all working examples of the Grensing et al. patent have beryllium contents of at least 62 wt. %. Also note the express disclosure in the Nachtrab et al., patent mentioned above that beryllium must be present in an amount of at least about 60 wt. %.
Two desirable properties of beryllium aluminum alloys are low coefficients of thermal expansion and high thermal conductivities. In this regard, all three commercial beryllium aluminum investment casting alloys mentioned above have coefficients of thermal expansion of about 14.5 μm/m (ppm) or less. In addition, two of these alloys have thermal conductivities of about 105 to 110 W/M-° K @30° C., while the third has a thermal conductivity of about 180 W/M-° K@30° C.
Beryllium is expensive. Therefore, it would be advantageous if the concentration of beryllium in these alloys could be reduced without adversely affecting their thermal or investment casting properties. And it would be especially desirable if this could be done in such a way that one or more of these thermal properties were actually improved.